The present invention relates generally to the field of electrical resistance welding and more particularly to methods and apparatus for making intercell welds in lead acid electric storage batteries.
A typical lead acid electric storage battery generally includes a container which is separated into a plurality of compartments by a series of partitions. A cell group is positioned within each of the compartments, the cell groups being appropriately interconnected to complete the assembled battery.
Each cell group typically comprises a series of interleaved positive and negative plates, with respective positive and negative plates being electrically interconnected by straps extending therebetween. To form the battery, these straps must be interconnected so that the positive strap of one cell group is electrically connected to the negative straps of an adjacent cell group. One way that this may be accomplished is by providing each of the partitions with an aperture, and by providing each of the battery straps with an upstanding lug positioned so that the adjacent lug pairs can be connected to each other through the aperture of the partition using any of a variety of techniques. One technique which can be used for interconnecting cell groups is resistance welding. One example of this technique may be found, for instance, in U.S. Pat. No. 4,166,210.
Generally, the technique illustrated in U.S. Pat. No. 4,166,210 calls for flat outer surfaces of the battery lugs to be positioned adjacent the partition and substantially enclosing the aperture. Thereafter, a pair of weld jaws are positioned adjacent the exposed surfaces of the battery lugs so that electrodes attached to the weld jaws can extrude portions of the lug material into the aperture of the partition. Upon achieving contact of the lug material, an electric current is directed through the weld jaws, the electrodes and the battery lugs. A strong and efficiently produced weld results, providing the desired intercell connection.
The foregoing technique has been found to work very well in providing effective intercell welds. However, despite the improvements afforded by such a technique, it has been found that a number of batteries still must be rejected for failure to achieve a proper intercell weld. Since a typical automotive lead acid electrical storage battery generally includes five such welds, the problem is multiplied because failure of any one of these welds can result in rejection of the entire battery. For this reason, continued attempts have been made to further refine this intercell weld technique.
One variable which has been found to have a significant effect on weld performance is the contact resistance between the extended material of the lugs prior to the application of the electrical welding current. This contact resistance is itself a function of variables such as contact area and surface conditions of the lug material. Since the lug material is normally lead, the surface conditions are affected by the presence of lead oxide as a result of oxidation, which increases electrical contact resistance. The contact area is a function of hardness of the lead material, the pressure exerted on the lugs by the weld jaws, as well as the shape of the extruding electrodes. Consequently, a number of variables must be precisely controlled in order to consistently attain the desired contact resistance. If the initial contact resistance is too high, hot spots and blow outs can occur thereby creating a faulty weld. On the other hand, if the contact resistance is too low, cold welds can result which are also unacceptable.
It is therefore desirable to develop a method and apparatus for precisely determining the electrical resistance between the contacting lug material prior to the application of the electrical weld current in order to reliably and consistently produce quality intercell welds.